1. Field of the Invention
This invention relates to an output torque control system for internal combustion engines for vehicles, which controls output torque from the engine so as to reduce a shock generated during gear shifting of an automatic transmission of the engine.
2. Prior Art
Conventionally, there is known a throttle valve control system for internal combustion engines for vehicles, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 5-321707, which changes the output torque from the engine in order to reduce a shock generated during gear shifting of an automatic transmission of the engine.
According to the known throttle valve control system, when the automatic transmission is upshifted, a throttle actuator is controlled to regulate the opening of a throttle valve of the engine such that the output torque from the engine is once increased in a so-called torque phase of the transmission and then the engine output torque is decreased to a value smaller than a value assumed before the upshift, in a so-called inertia phase of the transmission. As a result, a shock caused by a drop in the driving force of the vehicle in the torque phase and a subsequent shock caused by a sharp rise in the driving force in the inertia phase can be suppressed during the upshift.
FIG. 1 shows a timing chart useful in explaining a manner of changing the output torque from the engine by the throttle valve control, according to the prior art. As shown in the figure, when a command for gear shifting from a third-speed position to a fourth-speed position of the transmission (upshift) is issued, the throttle valve opening TH is progressively increased during an early stage of the upshift, based on a torque correction amount DTESFT which is then set to a value for increasing the engine output torque, whereby the actual engine output torque is increased according to the increased throttle valve opening TH. During a later stage of the upshift, the throttle valve opening TH is decreased to a value smaller than a value assumed before the start of the upshift, based on the torque correction amount DTESFT which is then set to a value for decreasing the engine output torque, whereby the actual engine output torque is decreased. Thus, the engine output torque can be smoothly changed, to thereby reduce a shock generated during the upshift.
The clutch engaging force of the transmission which determines the amount of torque transmitted through the transmission (hereinafter referred to as "the clutch torque") is determined mainly by hydraulic pressure applied on a clutch of a formerly selected speed position (released clutch) and the friction coefficient of the released clutch, as well as hydraulic pressure applied on a clutch of a target speed position (engaged clutch) and the friction coefficient of the engaged clutch. Normally, the clutch torque is sufficiently larger than the output torque from the engine. Therefore, even if the engine output torque is increased, e.g. by controlling the throttle valve, the engine output torque is transmitted from a main shaft of a gear mechanism of the transmission through the selected speed clutch to a counter shaft of the gear mechanism.
According to the conventional throttle valve control system, however, the engine output torque is increased on the premise that the clutch torque is always larger than the engine output torque. Consequently, if the clutch torque falls below the engine output torque, the following inconvenience is incurred:
FIGS. 2 to 5 are timing charts showing the relationship between various parameters assumed during an upshift according to the prior art, in which FIG. 2 shows changes in the parameters assumed during an upshift when the clutch torque is normal, while FIGS. 3 to 5 each show changes in the parameters assumed during an upshift when the clutch torque is abnormal.
As shown in FIG. 2, when the clutch torque is normal, it is sufficiently larger than the engine output torque.
FIG. 3 shows a case where an upshift is carried out, when the clutch torque lowers because the friction coefficient .mu. of the selected clutch lowers due to aging or deterioration of the clutch. When the friction coefficient lowers, the curve indicative of the clutch torque generally shifts downward as viewed in the figure such that it can be lower than the curve indicative of the engine output torque during the upshift.
FIG. 4 shows another case where an upshift is carried out, when the hydraulic pressure for operating the target speed position clutch rises with a delay. When the transmission has been continuously held in a parking position or in a neutral position over some period of time, the hydraulic oil for operating the formerly selected clutch is progressively drained from an oil passage for the released clutch. Once the hydraulic oil has been completely drained out of the oil passage, it takes a considerable time period for an oil passage for the engaged clutch to be filled with the hydraulic oil, resulting in a delayed rise in the hydraulic pressure for operating the clutch. Also when the hydraulic oil is low in temperature and high in viscosity, the hydraulic pressure for operating the clutch rises with a delay, similarly. If the hydraulic pressure rises with a delay, the clutch torque during an upshift can decrease below the engine output torque, as shown in the figure.
FIG. 5 shows a further case where an upshift is carried out, when the hydraulic pressure for operating the formerly selected clutch is quickly drained. If the hydraulic oil is high in temperature and low in viscosity, the hydraulic pressure for operating the formerly selected clutch is quickly drained. Also when a valve for switching a restriction through which the hydraulic oil passes is defective, the drain speed increases. When the hydraulic pressure for operating the formerly selected clutch is quickly drained, the clutch torque can decrease below the engine output torque during an upshift, as shown in the figure.
When the clutch torque decreases below the engine output torque due to a decrease in the friction coefficient of the selected clutch, variations in the rise speed of the hydraulic pressure for operating the clutch or the like, there occurs a slip in the selected clutch, resulting in a rise in the engine rotational speed NE. The slip of the clutch causes increased generation of heat and hence degraded durability of the selected clutch. Further, the increased engine rotational speed NE can cause an increased gear-shifting shock.